The present invention generally relates to an apparatus for regulating the transmission of data within a data communication system and, in particular, relates to such an apparatus for the masterless control of data flow among a plurality of closely spaced peripheral interface devices each having a plurality of external peripherals interconnected thereto.
In modern communication networks a plurality of data transmit/receive elements, each capable of transmitting and receiving digital data signals, are interconnected via a common transmission medium for the exchange of data therebetween. Naturally, if more than one such element were to transmit data at the same time, excessive errors and data loss would occur and the network would be quite unacceptable. Hence, one major consideration in the design of such a communication network is the regulation, or allocation, of the transmission time among the elements in a fair and efficient manner so that the users connected thereto are not subjected to undue delays or other inconveniences when attempting to transmit information. This consideration persists regardless of the magnitude of the network designed, i.e., a long-haul network, a local area network, or a short-haul local area network.
At the present time and, as anticipated for years to come, one of the primary systems of interest for many users is a local area network or a short-haul local area network. Such a network is generally employed in a geographically concentrated facility, such as a university or business site, and includes a plurality of nodes each having a number of peripheral devices connect thereto. With the advances in semiconductor technology, the control of such nodes is presently available on a large-scale integrated circuit chip. These nodal chips can then be interconnected to provide communication services with other similar nodes to form a network. This design consideration can be characterized as the avoidance of collisions between two or more users attempting to simultaneously transmit information over a common transmission medium which interconnects a plurality of nodes.
One conventional solution to this congestion, or collision, problem is the use of a carrier signal that is transmitted to each node of the network and which is detected thereat.
The most common scheme is known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD). In such a scheme, the phrase "carrier sense" means that any data transceiver client wishing to transmit, "listens" first. If the communication medium is being used by another subscriber, the client defers transmission. "Multiple access" indicates that any client wishing to transmit can do so without the need for a central controller. "Collision detection" refers to the fact that when the communication bus is idle any element can begin transmitting.
The CSMA/CD scheme is implemented such that during the transmission of data by any party this carrier signal is present. The presence of the carrier signal on the common communication medium serves to indicate to other nodes attempting to transmit that to do so would create a collision. Consequently, when the carrier signal is detected by any node attempting to transmit, that node is restrained from transmitting. This restraint is frequently implemented by requiring an enabling signal to be provided before actual data transmission occurs. Upon failure to receive such an enabling signal, due to the detection of a carrier, transmission is restrained.
The carrier signal, even for local area networks such as a large building, or a cluster of buildings, is most frequently transmitted to every node via a coaxial cable. Such a solution is quite expensive since the coaxial cable needs to be routed to each station along the network and requires expensive interface equipment.
From the above, it is apparent that schemes such as CSMA/CD for avoiding simultaneous transmission collisions become expensive due to the requirement of extra cable lengths and hard wire interconnections to service every node.